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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A27 much as 40 ft. Head declines in the lower pumped zone in During the 1960's and 1970's, the annual decrease in the Sacramento and northern San Joaquin Valleys gen- ground-water storage was about 800,000 acre-ft, repre- erally are small—less than 80 ft(fig. 18). However,in the senting about 7 percent of annual pumpage(fig. 16). The heavily pumped western and southern parts of the San long-term decrease in aquifer storage of 60 million Joaquin Valley, heads have declined from 100 to 400 ft acre-ft,although very large,represents only a small part since development began. Since the late 1960's, the of the more than 800 million acre-ft of freshwater stored increased availability of imported surface water in these in the upper 1,000 ft of sediments in the Central Valley. areas and the accompanying decrease in ground-water Nevertheless, the lowering of water levels in the upper pumpage has stopped the long-teem decline and allowed and lower zones caused a significant increase in pumping some recovery of ground-water levels. Year-to-year lifts and thus a significant increase in the cost of pumping changes in ground-water levels have reflected the avail- ground water. During the early 1980's, ground-water ability of surface water. During wet or average years, pumpage decreased, ground-water levels rose in many more imported surface water is available fur irrigation;as areas, and there was virtually no further decrease in a result, well pumpage decreases and ground-water ground-water storage. levels rise. During drought years,such as 1976 and 1977, less surface water is available, wells are more heavily IANC SUBSIDENCE pumped, and ground-water levels decline. When heads have declined sufficiently in the lower The largest volume of land subsidence in the world pumped zone for inelastic compaction of clay beds to occur, the rate of water--level decline slows. This slower caused by human activities is in the Central Valley. The rate results because the effective storage coefficient is area affected by subsidence includes much of the south- significantly increased (Williamson and others, 1889). ern part of the San Joaquin Valley and smaller areas in far,t the Sacramento Valley and the Delta( g. 19). S The result of the decline in ground-water levels from (fig. 19). the start of development until 1977 has been the loss of an largest volume of land subsidence is caused by ground- estimated 60 million acre-ft of aquifer storage. This Fater pumpage and the resulting compaction of clay in depletion of storage is made up of three components. the San Joaquin Valley. However, other processes have contributed to land subsidence locally as described by Poland and Evenson (1966) and Poland and Lofgren 1. Long-term lowering of the water table that results (1994)_ Briefly, the five processes that cause subsidence from dewatering of the shallow sediments-40 rail- are. lion acre-ft, 2. Inelastic compaction (permanent reduction of pore 1. Compaction of fine-grained sediments in the aquifer space)-17 million acre-ft. system resulting from head declines due to heavy 3. Elastic storage (compression of sediments and ex- ground-water pumpage. pansion of water)-3 million acre-ft. 2. Compaction of sediments in petroleum reservoir rocks caused by oil and gas extraction. The changes in storage were calculated with the 3• Hydrocompaction—compaction of moisture-deficient computer model as described in chapter ❑ (Williamson sediments following the first application of water. and others, 1989). The decrease in storage due to 4. Compaction of peat soils following land drainage. dewatering is the product of specific yield, water-table 5. Tectonic subsidence. decline, and area of the shallow aquifer dewatered. Compaction of peat soils and subsequent land subsid- Similarly, the change in elastic storage is the product of ence has occurred in an area of about 450 rni2 in the Delta elastic specific storage, thickness of confined aquifer, area formed at the confluence of the San Joaquin and head decline, and area of aquifer affected. Sacramento Rivers (Poland and Evenson, 1966). Islands The loss of storage from inelastic compaction of clay in the Delta area that were originally at or slightly above beds causes a permanent loss of pore space that in turn is sea level are now 10 to 20 ft below sea level. This type of balanced by an equivalent volume of land subsidence(see subsidence results from oxidation and compaction of peat next section). Extensive leveling by the National Geo- soils following drainage of marshlands for agriculture_ detic Survey over many years has established the extent This area was drainer]in the middle and late 1800's,which of land subsidence in the Central Valley (Poland and resulted in subsidence that is continuing today.Increased others, 1976; Ireland and others, 1984). The 17 million pumping is required to maintain a lowered water table for acre-ft of storage loss attributed to inelastic compaction cultivation. of fine-grained sediments is simply the volume of land Hydrocompaction refers to the compaction of moisture- subsidence derived from these surveys, deficient deposits above the water table following the